1. Field of the Invention
The invention relates to methods for recovering oil from an oil reservoir.
2. Description of the Related Art
Various techniques have been employed to recover crude oil from oil reservoirs (subterranean oil-bearing formations). Primary recovery techniques usually rely on natural reservoir pressure to effect withdrawal or extraction. Secondary recovery techniques typically entail increasing reservoir pressure by introduction (injection) of energy by flooding with water, steam, or gases at a location other than the wellbore but usually in the general direction of the wellbore. When water is introduced, it is referred to as flooding. Oil is then recovered at the wellbore. Tertiary recovery usually entails enhancement of recovery by more sophisticated techniques, such as heating, Theological modification of flooding fluids, and modification of formation and/or pore geometry.
One primary recovery technique involves injecting water through a wellbore into a reservoir to open or fracture it to an extent to yield additional oil. Water may also be used to carry a proppant, such as sand, to aid in keeping open the pores and fractures in the formations after release of water pressure to further enhance oil yield.
In tertiary recovery, fluids of aqueous surfactant solutions/dispersions have been injected during flooding. Certain anionic and nonionic surfactants, such as alkylphenol polyglycol ether sulfonates, oxyalkylated aliphatic alcohols and oxalkylated alkylphenols, have been disclosed as useful for this purpose. The surfactants lower the interfacial surface tension (IFT) of the fluid and enable it to more easily form emulsions mixtures and/or microemulsions with the oil in the reservoir. The formation of mixtures and/or microemulsions dislodge the entrapped oils in the formations through IFT reduction and solubilization of oil in the aqueous surfactant solutions thereby increasing the recovery of oil from subterranean formations. However, the surfactant fluids have less viscosity than the oil, so their effectiveness in pushing the oil from the formations is limited.
To more effectively push oil out of subterranean formations, the viscosity of flooding fluids has been increased by the addition of high molecular weight polymers, such as polyacrylamide (PAM). Such high molecular weight polymers have also been used in combination with conventional anionic surfactants.
A drawback with using conventional anionic surfactants alone in flooding fluids is that their primary physical action is reduction of IFT without significant enhancement of viscosity. A drawback with the use of high molecular weight polymers alone is that they do not reduce IFT. Combining conventional anionic surfactants with high molecular weight polymer to achieve both IFT reduction and viscosity increase requires the use of two different components, which can be expensive and cumbersome. Further, some conventional anionic surfactants can have undesirable interactions with high molecular weight polymers and counteract viscosity enhancement. Still further, high molecular weight polymers commonly adsorb or precipitate out in reservoirs causing damage to the formations and diminution of oil recovery.
It would be desirable to have a flooding fluid and method for use in tertiary oil recovery that employed a single additive or component that effected a significant decrease in IFT and a significant increase in viscosity for the flooding fluid.